Oracle学习笔记<<2>>
Oralce Architectural Components
Meroy Structure
    Oracle’s memory structure consists of two memory areas known as :
    System Grobal Area(SGA):allocated at instance start up,and is  fundamental component of an Oralce instance
    Program Global Area(PGA):allocated where the server process is started


SGA:System Grobal Area
    Is dynamic,sized
    Size by the SGA_MAX_SIZE parameter
    Allocated and tracked in granules by SGA components
        Contiguous virtual memory allocation
        Granule size based on total estimated SGA_MAX_SIZE
    SGA可以动态改变，最大的尺寸可以通过SGA_MAX_SIZE来改变。Granules是SGA的最小单位。SGA是连续的内存区域，并且最大不能超过  SGA_MAX_SIZE.
    可以通过show sga 查看SGA 大小.
    可以通过语句
     select component,granule_size from v$sga_dynamic_components.
    查看granule的大小

    1．The SGA consists of several memory structures:
       Shared Pool: SHARED_POOL_SIZE
       Database Buffer Cache:  DB_CACHE_SIZE
       Redo Log Buffer: LOG_BUFFER
       Other structures(for example ,lock and latch management,statistical data)
    2．There are two additional memory structures that can be configured within the SGA:
      Large Pool: LARGE_POOL_SIZE
      Java Pool: JAVA_POOL_SIZE

Shared Pool
    Used to store
        Most recently executed SQL statements
        Most recently used data definitions
    It consists of two key performance-related memory structures:
        Library Cache
        Data Dictionary Cache
    Sized by the parameter
    每一个SQL将被解析，制定执行计划。解析SQL语句被存到共享池中。包含两部分重要内容。为Library Cache和Data Dictionary Cache


    Library Cache
    Stores information about the most recently used SQL and PL/SQL statements
    Enables the sharing of commonly used statements
    Is manager by a least recently user(LRU) algorithm
    Consists of  two structures:
          Shared SQL area
          Shared PL/SQL area
    Size determined by the Shared Pool sizing
    存储的是经过编译解析后的SQL语句和PL/SQL语句的内容.
    包含两部分。一部分是SQL area ,一部分是PL/SQL area
    大小由Shared Pool 管理

Data Dictionary Cache
    A collection of the most recently used definitions in the database
    Includes information about database files, tables , indexes , columns , users, privileges ,and others database objects.
    During the parse phase ,the server process looks at the data dictionary for information to resolve object names and validate access
Improves response time on queries and DML
    Size determined by the  Shared